The Swiss embassy in Tehran, which hosts the US interests section in the Islamic republic, passed a letter from the Iranian government to US officials on Friday outlining Iran's formal complaint over accusations that military factions inside the regime were directly linked to the alleged plot.

"In a recent letter to the US government, Iran has insisted that American authorities must publicly apologise to the Iranian government and its citizens for the false accusations they publicised against Tehran in violation of international norms and regulations," Iran's state-run Press TV reported.

US officials claimed to have uncovered an Iranian plot to assassinate Adel al-Jubeir in a sting operation involving the FBI and the Drug Enforcement Administration.

The US said Iran was attempting to hire a member of a Mexican drug cartel to carry out the killing but failed because the gang member was actually a US informant.

The main suspects involved in the alleged plot were identified as an American-Iranian, Manssor Arabsiar, 56, who was arrested at New York's JFK airport in late September, and Gholam Shakuri, an Iranian who remains at large in Iran, according to the US.

The US version of events has been met with scepticism both from sympathisers of the Iranian regime and its opponents.

Despite this, the US continues to back its claims and has lobbied Europe for tougher sanctions against Iran in response to the alleged plot.

David Cohen, the US Treasury undersecretary for terrorism and financial intelligence, was in London last week seeking support from his British counterparts. "Iran needs to be held accountable for this plot," he said.

Iran has tried to distance itself from the affair by pointing the finger at an Iranian dissident group, the People's Mujahideen of Iran (MEK) and accusing the US of fabricating the allegation in an attempt to damage Iran's international reputation.

An Iranian diplomat who spoke to the Guardian on condition of anonymity said the US has resorted to its "entrapment technique" in order to smear Iran in the eyes of the international community.

"Americans have not presented any compelling evidence whatsoever to Iran to prove the plot was hatched by the Islamic Republic. Neither have they provided consular access to Mr Arbabsiar," said the diplomat.

Arbabsiar pleaded not guilty last week to charges that he was involved in an assassination plot.

The Iranian diplomat claimed entrapment techniques have been used previously by the US in cases against individuals accused of attempting to smuggle or export military components to Iran.

In one recent case, in October 2006, the former Iranian ambassador to Jordan, Nosratollah Tajik, was arrested in Britain for allegedly procuring night-vision goggles for Iran. He is currently under house arrest in Britain awaiting extradition to the US.

"This plot, which was discovered this time again by an 'undercover American agent', is yet another [entrapment] scenario painted by the US government to impede growing Iranian progress and make trouble for the country," added the diplomat.

In an attempt to reduce tensions with Saudi Arabia, Iran's foreign minister Ali Akbar Salehi travelled to Riyadh last week to attend the funeral of Crown Prince Sultan bin Andul-Aziz.